1. Field of the Invention
The present invention relates to a decompressor suitable for reducing a load on an engine starting system.
2. Description of Background Art
When starting an engine by forcibly rotating the crankshaft of the engine to introduce fuel into a combustion chamber and to produce a high pressure in the combustion chamber by a piston connected to the crankshaft by a connecting rod and by igniting a mixture contained in the combustion chamber, it sometimes occurs that the piston is moved by the pressure in the combustion chamber to force the crankshaft to rotated in the reverse direction. In such a case, a torque acts on the crankshaft in the reverse direction and increases the load on a starting system.
A decompressor previously proposed to reduce a load on the starting system opens the exhaust valve forcibly when the crankshaft reverses to reduce the pressure produced in the compression chamber by compressing the mixture so that the fuel supplied into the combustion chamber may not be ignited. A reduction of the pressure in the combustion chamber is called xe2x80x9cdecompression.xe2x80x9d
Such a decompressor is disclosed in, for example, Japanese Utility Model No. H4-52413, xe2x80x9cAutomatic Decompressor for Four-Stroke Cycle Engine.xe2x80x9d As shown in FIGS. 8 and 9 appended to the specification for Japanese Utility Model No. H4-52413, a four-stroke cycle engine has a positioning plunger 54 engaged with an engaging member 52.
According to the prior art, the positioning plunger 54 is pressed by a coil spring into engagement with the engaging member 52. Consequently, a decompression cam 50 is pressed against a camshaft 12 by the resilience of the coil spring and hence the stationary decompression cam 50 exerts a frictional resistance on the rotating camshaft 12. This frictional resistance causes a loss in the output of the engine. Therefore, it is desired to reduce the frictional resistance to the least possible extent.
Although the plunger 54 is maintained in contact with the decompression cam 50 at all times by the coil spring, sometimes, the plunger 54 and the decompression cam 50 are caused to vibrate and strike against each other by vibrations generated by the engine, so that noise is generated.
Accordingly, it is an object of the present invention to provide a decompressor capable of suppressing the loss in the output of the engine and for preventing the generation of noise.
To achieve this object, according to a first aspect, a decompressor for a fourstroke cycle engine has a decompression cam supported on a camshaft interlocked with a crankshaft by an overrunning clutch to forcibly open an exhaust valve wherein the overrunning clutch has an outer member provided on its outer circumference with a projection, a cylinder head supporting the camshaft thereon is provided with a stopper, and the decompression cam is held stationary while the camshaft is rotating in a normal direction by contact between a side surface of the projection and a side surface of the stopper.
The decompression cam is held stationary while the camshaft is rotating in a normal direction by contact between a side surface of the projection and a side surface of the stopper.
Therefore, any radial pressure is not exerted on the overrunning clutch by the stopper and the overrunning clutch does not apply any pressure to the camshaft. Consequently, frictional resistance that acts against the rotation of the camshaft can be reduced and the loss in the output of the engine can be suppressed.
Since the respective side surfaces of the projection and the stopper merely slide relative to each other even if the engine generates vibrations, noise generation by the projection and the stopper can be prevented.
According to a second aspect, the overrunning clutch comprises the outer circumference of the camshaft, a annular member is loosely positioned on the outer circumference of the camshaft so as to define an annular space between the outer circumference of the camshaft and the annular member. Rollers are disposed in a space defined by grooves formed in the inner circumference of the annular member and the outer circumference of the camshaft. The rollers are capable of exercising a wedge action.
In the overrunning clutch the annular member is provided in its outer circumference with recesses for lightening or reducing the weight in sections between the grooves.
Thus, the annular member has a small moment of inertia and the annular member is capable of quickly starting rotation when the camshaft reverses.
Consequently, the response to open the exhaust valve can be improved.
According to a third aspect, the stopper is inserted in a hole formed in the cylinder head from the side of the camshaft and is held in place with a retaining ring. The stopper is held in the hole formed in the cylinder head from the side of the camshaft with a retaining ring.
Therefore, when attaching the stopper to the cylinder head, the stopper can be moved in the same direction as that in which the camshaft and other parts are moved when attaching the same to the cylinder head, and work for attaching the stopper to the cylinder head and work for forming the hole in the cylinder head can be facilitated.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.